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Can I Compost Water?

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Moisture is an essential part of composting – but not all water should be composted.


What types of water NOT to add

  • Soapy water from baths/showers or washing cars
  • Dishwater from washing up
  • Washing machine waste water
  • Waste water from the toilet
  • Stagnant water
  • Flood water
  • Chlorinated water from swimming pools or hot tubs
  • Water contaminated with bleach or other cleaning agents
  • Water contaminated with petrol or oils
  • Water contaminated with paint

Basically, don’t compost any water that has been mixed with anything on the “don’t compost” list.

Soaps and other chemicals are bad news for a compost heap. They will kill the delicate microbes that are busy doing the composting work — most of the cleaning chemicals and soaps we use around the home are purposely design to kill bacteria and the like but in a compost heap, those guys are our friends!

Whatever chemicals or other pollutants are in the water will also contaminate the resulting compost – possibly not such a big problem with a bit of soapy water from washing your hands but more of a problem for things like petrol, paint or fecal matter — not things you want getting into your veg bed!

Stagnant water, even if it was clean to start with, should not be composted because of the risk of Legionnaire’s Disease and because it also can inhibit our friends, the composting microbes.

Flood water also shouldn’t be composted – unless you know for sure that it hasn’t come into contact with any nasty substances. (Flood water can pick up all sorts of petrol-y substances from roads and in a bad flood, sewerage can flow into river courses etc.)


What CAN be added

  • Rainwater
  • Water from cooking that hasn’t come into contact with too many fats
  • Water from washing veg including any peelings
  • Cooled boiled water from the kettle etc
  • Pond water including algae
  • Water from kids’ paddling pools as long as it’s not soapy
  • Muddy water from washing the dog or other muddy things!
  • Melted ice from defrosting the freezer or snow
  • Unwanted water cooler/fountain water
  • Leftover watery drinks like tea, coffee or juices (but nothing overly milky)

Basically, any water that is either clean or only mixed with things that can be composted.

Water from cooking – such as from boiling pasta or vegetables – can be added but only if it is free from oils (cooking, butter or animal fats) as they can attract vermin to the pile. (You can also use that water in stocks, soups and baking – see our water page on Recycle This for more information and ideas.)


How to add water to a compost heap

As mentioned at the start, moisture is an essential part of the composting process. Some people choose to leave their bins or heaps open so they can soak up rainwater as they go, but if you’ve got a lid or a covering, you will probably needed to add some liquid at some time or another (especially if you’re adding a lot of browns)

When adding water to a compost bin or heap, add it to the centre of the pile and stir it through if you can to get the maximum benefit – that’ll stop it simply dampening the top layer and make sure the often-warmer middle won’t dry out.

Do try not to over-wet the heap though. Overall, the compost should be moist not wet. This is particularly important if you’re using a sealed compost system – most purpose-bought compost bins have drainage holes (or a completely open bottom) to allow excess water to drain out.

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